Stacy Kopso, M.Ed.,RT(R)(M). Control the amount of radiation reaching the image receptor by terminating the length of exposure Determines the exposure.

RADIOGRAPHIC EQUIPMENT

Stacy Kopso, M.Ed.,RT(R)(M)

Automatic Exposure DevicesAEC or Phototiming

Control the amount of radiation reaching the image receptor by terminating the length of exposure

Determines the exposure time (therefore, total exposure)

Reproduce a desired amount of film density on images taken of selected body parts regardless of changes within the patients being examined

Designed to maintain consistent radiographic densities on images taken on wide variety of patients

Automatic Exposure DevicesAEC or Phototiming

Advantage ReproducibilityRadiographs taken on a mixed population of

patients will fall within an acceptable or diagnostic range

Reduces repeats due to technical factors Disadvantage

Improper positioning of central ray or bucky alignment

Incorrect photocell selection

Automatic Exposure DevicesAEC or Phototiming

Items that impact the AEC at the control panel kVp mA Backup timer Density selector Photodetector (cell)

Items that impact the AEC at the x-ray table Alignment of CR to part of interest Alignment of bucky tray to CR

Automatic Exposure DevicesAEC or Phototiming

Items that impact the AEC at the control panel kVp mA Backup timer Density selector Photodetector (cell)

Items that impact the AEC at the x-ray table Alignment of CR to part of interest Alignment of bucky tray to CR

kVp & mA Selector

kV selector Reduce dose (increase kVp, to decrease mAs) Penetration Contrast

mA selector “rate of exposure” Change “speed” or “length of time”

Backup Timer

Maximum length of time the x-ray exposure continues when using AEC

Safety measure for patient and tube Set by radiographer or in unit Maximum termination 600 mAs Should be set at 150% to 200% of

expected exposure time

Density Adjustment

The predetermined exposure level needed to terminate the timer is increased or decreased

Change rate of exposure Increase or decrease the amount of radiation

produced based on size of patient +or-1=25% +or-2=50%

When would we need to use the + or – Pediatric if pt doesn’t extend out to the cells on a cxr Hip xray if pts belly is big and you need to penetrate through

to see head of femur Routinely having to use this setting indicates a

problem (AEC)

Radiation Detectors

Radiation is transmitted through the patient & converted into an electrical signal, terminating the exposure time

Over/under expose patient if wrong photocell is selected or patient is not positioned over cell

Predetermined level of radiation AEC uses Ionization chambers or

Phototimers Sensors, chambers, cells or detectors

Phototimer

Phototimers Earlier generations Use a fluorescent (light-producing) screeen

and a device that converts the light to electricity

Photomultiplier tube Electronic device that converts visible light

energy into electrical energy Photodiode

Solid-state device that converts visible light energy into electrical energy

Phototimer

Exit device Detectors are positioned behind the image

receptor Radiation must exit the image receptor before it

is measured by the detectors Light paddles coated with fluorescent material

serve as detectors The radiation interacts with paddles and

produces light Light is transmitted to PM tubes or photodiodes

that then convert light into electricity The timer is tripped and exposure is terminated

Phototimer

Ionization Chamber

Most commonly used today Cell that contains air and is connected to the

timer circuit via electrical wire Entrance device

Detector positioned in front of the image receptor When chamber is exposed to radiation from a

radiographic exposure, the air inside the chamber becomes ionized, creating an electrical charge

Charge travels along wire to timer circuit Timer is tripped and exposure is terminated

Ionization Chamber

Minimum response time

Minimum response time Shortest exposure time that the system can

produce 1ms Increase the mA to decrease the exposure

time(pediatric) Make sure the minimum response time is not

longer than the amount of time selected (overexposure)

Pediatric manual techniques

Manual Exposure

Exposure factors are determined by the size or thickness of the part under examination

Require the use of carefully constructed technique charts

APR Anatomically programmed radiography Table top x-rays

Quality Control AEC

When unit is first installed and at intervals thereafter

When there is a change in IR (CR to DR) Consistent need to adjust density controls

indicates that the AEC needs to be recalibrated

Use of phantoms to show consistent exposures to the image receptor for variations in technique factors, patient thickness and detector selection

Beam Restriction Devices

Aperture diaphragm Lead plate w/ hole cut in middle that is slightly

smaller than the image receptor size

Beam Restriction Devices

Cylinders and Cones Attaches to the bottom of the collimator

housing Creates a circular shaped collimated field Maximum beam restriction occurs w/an

increased length and decreased diameter of cylinder or cone

Beam Restriction Devices

Variable Aperture Collimator Controlled by radiographer Permits adjustment of length & width of

radiation light field Utilized light-localization device to estimate

the size and shape of field

Beam Restriction

Collimation- decrease in the size of the projected radiation field Pt dose

decreases Scatter radiation

decreases Contrast

increases Film density

decreases Digital: Quantum noise

increases Increasing collimation & field size

decrease Decreasing collimation &increasing field size

increase